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Originally Posted by WVGasGuy
I thought I put this to rest last week but after a 200 mile drive I've got it straight in my head (I think).
Using my Garmin iQueM5 finally on a trip I found that in 198 miles (by GPS reading) of driving my odometer read 195.1.
Also driving at 70 miles per hour (by my speedometer) my Garmin was reading in the range of 68.5 miles per hour.
That's why I think I confused myself last week. The errors are divergent.
The Speedometer reads too HIGH. To solve the speedometer error you would have to have a taller tire/wheel combo which would register a slower speed than the OEM's at the same speed, since it takes less rotations of the tire to correct the "too fast" speedometer reading.
However, since the odometer reads LOW you would need a shorter tire/wheel combo to increase the number of rotations per mile over the OEM's to correct the error.
The speedo error is reasonable and it keeps you under the speed limit and is a decently close margin of error. However the odometer error is factored into the FE readings and thus we probably are off by 1.5% (too high) on actual FE calculations. (Still it's pretty close too_not as bad as 5% as some have guessed at)
All in all both were pretty close. But a simple tire/wheel change can't solve both problems since the errors diverge as you make changes.
No responces necessary as I wore most of you out last week on this. I'm just clearing my head.
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My observations are the same as yours, which I've reported many times in here. My speedo appears to report 5% too high (reports 60mph when I'm really doing 57mph) and the odometer reports too low (which is why my FE calculations are 2% less than what the OBD is telling me). I'm more concerned with the speedo problem, since the highway patrol could care less what the odometer reading is!
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Originally Posted by ralph_dog
I would think that the GPS is less accurate than the odo. At best, a GPS is only accurate to within 9 feet unless you have the secret military codes????
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Yes, but that is relative to the satellite at any given point. If you are off by 9ft from where you truly are "here", then you are also off by 9ft from where you are truly are 500 miles away. It's not like you get a 9ft deviation at every point of your journey. So at the extreme, the GPS odometer would be off by 18ft (you are the furthest away from true location at both ends of your journey) which is statistically inconsequential after 500 miles.